1. Field of the Invention
The invention relates to methods and apparatus for causing the mixing of substances and fluids at interfaces with varying density gradients. More particularly the invention describes a method of generating precisely-defined wall shearing stresses at a substrate/fluid interface, especially those of a sediment/water interface within a confined space, together with an apparatus to realize this method.
2. Description of the Prior Art
Wall shearing stresses play a pivotal role in hydrodynamical, sedimentological, biological, geochemical, and engineering process and their control at substrate/fluid interfaces. The investigation of sediment/water boundary layers in rivers, lakes, and oceans is of great importance; e.g., the determination of the vertical exchange (flux) of chemical substances between sediment and overlying water, or the determination of porewater concentration profiles as well as the reservoir capacity of sediments for chemical substances including adsorbed pollutants. The latter concerns waste disposal, pathways of toxins and environmental health. In order to achieve realistic results in these areas, it is mandatory to establish quantitatively the link between the studied process and the wall shearing stresses at the substrate/fluid interface. To achieve this goal a flow pattern in the fluid has to be generated which produces precisely known wall shearing stresses in the substrate/fluid boundary layer.
The generation of a boundary layer structure homogeneous over a relatively large surface area between a substrate and overlying fluid with a resulting spatially homogeneous distribution of the wall shearing stress over the whole area can be relevant also for industrial processing and manufacturing techniques, especially in microbiological manufacturing techniques, titration processes, surface coatings, and others. It is also useful in establishing the best possible growth environment for bacterial cultures or to optimize their exudates.
The invention has thus the task to find an engineering procedure as well as an apparatus which permits the generation of precisely defined wall shearing stress fields in a substrate/fluid boundary layer within a confined measuring volume for steady and variable time histories.
"Precisely defined" wall shearing stresses means that their magnitude generated at any point and time on the surface of the substrate in the confined measuring volume is known within a measuring uncertainty (mean error) of less than 10%. Depending on desired application, these distributions can have different spatial and temporal features as hereafter shown.